It has been shown that a functional gene A product is required in order for SV40-transformed rodent cells to express many characteristic properties of the transformed state. A series of temperature-dependent cells are available, as are transformed hamster cells which contain SV40 T antigen in the cytoplasm. The availability of such marked cell lines and of pertinent technical expertise permit a sharply-focused investigation of the biological properties of SV40-induced proteins. The functional activities attributable to the SV40 gene A protein in transformed cells will be determined, the nuclear and cytoplasmic forms of the T antigen will be isolated and their properties compared, and the biological consequences of the introduction of partially-purified SV40 early proteins into normal eukaryotic cells will be examined. These studies will assign biological properties to the SV40 early proteins and will further define the relationships between the gene A product, T antigen, and conversion to the transformed state.